Heated calender rollers are especially used in the paper industry, rubber industry or plastics industry. In view of the very small thickness of processed webs, these calender roller are manufactured within exacting tolerances so as to provide same gap thickness and line pressures along the effective surface at working load. Rollers provided with cambering meet these requirements only by maintaining predetermined parameters. The bending of the rollers can be countered by employing suitable devices applying bending moments onto the roller journals. It has been found, however, that the loading or deflection of the generatrix could not be compensated by such bending devices uniformly over the entire surface length of the roller.
Consequently, it has been proposed especially in paper and textile calenders to use so called "floating rollers" which include a stationary shaft centering via suitable bearings a hollow cylindrical roller body which receives a pressure fluid introduced into a gap defined between the shaft and the roller body.
A drawback of such floating rollers is that their shafts have a smaller support width than the remaining rollers. Especially when using heatable rollers which are equipped with displacement filling practically all of the rollers except for the clearance traversed by the fluid, problems have been encountered upon centering and fixation of conventional displacing bodies of sheet metal within the hollow cylindrical roller bodies, and moreover at high speeds or sudden decelerations to a standstill, forces are generated which could considerably deform or destroy the displacing body thus requiring expensive and difficult repair work.
During the manufacture of paper, it is required to provide identical bending lines of the converging rollers. This could be obtained by cambering or roller flextion or by use of compensating rollers. A further adaptation of the bending lines is provided through partial cooling and/or heating of the rollers to thermally alter their diameter. In this connection the use of cold air or hot air or steam nozzles and also partial induction heating has been proposed.
Especially in the production of paper, it has been shown that the achieved smoothness, gloss as well as printability can be considerably enhanced when increasing the nip pressure and/or especially the roller temperature. Thus, it appears to be advantageous to heat up the rollers considerably so that the range of the thermal compensation is limited.
Practice has, however, revealed that a considerable amount of the margin of the paper webs had to be discarded because the paper web obtained during the stock preparation in the breast box, during the wire section, pressing section and drier section not only showed calibrating errors but especially due to the drying phase produces margins of the paper web that are heated to a higher degree than the central areas which remain in a more moist stage. Although the heat transmission from the roller to the paper is reduced at the margins because of the lower heat conductivity, the margins despite the lower heat absorption, remain drier and are heated up to higher temperatures than the moist central areas as the drier margins have a lower specific heat than the moist central areas.
Consequently, the calender rollers finishing the paper webs will emit less heat at the margins of the web than at the central areas so that its thermal balance and thus its outer contour is marred. This drawback is further worsened by the mechanical heat transformation which is increased at the presence of narrow gaps and additionally increased by the usually stronger margins of the paper.
To prevent a different drying degree from arising between the margins and the central areas of the paper, it has been proposed to moisturize the margins so as to obtain a uniform dampness. To provide such a damping is rather complicated and did actually not solve the described problems.